This invention relates to electronic displays and, more specifically, to a luminescence display tube for simultaneously displaying a plurality of digits, which may be numerals, symbols, letters, and/or the like.
Luminescence display tubes have also been referred to as a fluorescence display tube or a phosphor indicator tube. As is described below with reference to the accompanying drawing, a multi-digit luminescence display tube comprises a cathode and an anode assembly comprising a substrate and a plurality of luminescent anode segments formed on the substrate for each of the digits. The cathode structure may comprise either a common filament for the anode segments of all digits, or individual filaments for the anode segments of the respective digits. Alternatively, the cathode may comprise two filaments for the anode segments of all digits.
The tube further comprises a plurality of grids between the cathode and the anode segments and first means for supplying first electric voltages between the cathode and the grids and between the cathode and at least a selected one of the anode segments for making the selected anode segment or segments to luminesce. The grids are in substantial registration with the anode segments of the respective digits and may be said to spatially cover the respective digits. The first voltages comprise a grid voltage and an anode or plate voltage. For a sophisticated luminescence display tube, a plate voltage of about 50 to 60 volts was supplied to the selected anode segment or segments. Use is made of a lower plate voltage, for example, of 24 volts for a recent luminescence display tube. An integrated driving circuit operable with a power source of about 24 volts, is also employed to drive the recent type luminescence display tube.
On the other hand, it is the practice for a multi-digit luminescence display tube to superpose a second electric voltage on the first voltage supplied between the cathode and selected of the grids to prevent the anode segments covered by the selected grid or grids from luminescing. The second voltage, however, has had to be negative with respect to the cathode potential. This has made it necessary to use a separate bias source to furnish the second voltage in addition to a principal voltage source for the driving circuit for energizing the luminescence display tube, with the concomitant additional expense of the second source.
The critical value of the second voltage for suppressing the luminescence of the selected anode segment or segments may be termed a cutoff grid voltage. It has already been known that the negative cutoff grid voltage may be lessened with a reduction in the percentage of geometrical coverage of the grid, namely, the ratio of the total cross-sectional area of the grid wires to the area generally covered by the grid. This, however, reduces the brightness of the luminescence. The negative cutoff grid voltage may also be lessened with a reduction in the filament voltage. This, however, necessitates the use of a separate filament for the anode segments of each digit which undesirably increases the power consumption of the tube and the variations in the brightness of the respective digits.